Internal Rotation of the Methyl Group in the Electronically Excited State: o- and m-Toluidine

The fluorescence excitation and dispersed fluorescence spectra of jet-cooled o- andm-toluidine were observed. Vibrational analysis of the spectra provided us with thepotentials for the internal rotation of the CH 3 group in both ground and excited states. Ino-toluidine, a large potential barrier to the internal rotation in the ground state ispractically removed in the excited state. On the other hand, a nearly free internalrotation of the CH 3 group in the ground state of m-toluidine gains a large barrier by theelectronic excitation. The great change in the barrier height upon the electronic excitation is more remarkable than that found for fluorotoluene. A close relationship between the barrier height and the π electron density at the ring carbon atom was found, indicating the hyperconjugation as the origin of the barrier height in the absence of sterichindrance.